danfoss refrigeration basics

8/8/2019 Danfoss Refrigeration basics

1/24

Refrigeration

- an introduction to the basics

LectureREFRIGERATION &

AIR CONDITIONING DIVISION

MAKING MODERN LIVING POSSIBLE


2/24

2 DKRCC.PF.000.F1.02 / 520H0924 Danfoss A/S (RC-CMS / HBS), 01 - 2006

This Danfoss publication must be regarded as a supplement to the comprehensive

literature on refrigeration that is available today and which is primarily aimed atreaders with a professional relationship to the refrigeration industry/trade e.g. re-

frigeration engineers and installers.

The contents of this book are intended to interest those who are not engaged every

day with refrigeration plant but who wish to extend their knowledge on the basic

principles of appliances they see every day.

When compiling the material for the booklet a deliberate attempt was made to pro-

vide a thorough description of the elementary principles involved together with an ex-

planation in everyday language of the practical design of the individual components.

For additional training material we refer to

http://www.danfoss.com/BusinessAreas/RefrigerationAndAirConditioning

Choose Training & Education.

Nordborg, 2006

Refrigeration -an introduction to the basics


3/24

Danfoss A/S (RC-CMS / HBS), 01 - 2006 DKRCC.PF.000.F1.02 / 520H0924 3

Contents1. Introduction .......................................................................................................................................................................................... 5

2. Fundamental terms ........................................................................................................................................................................... 62.1 Unit systems .............................................. ..................................................... .................................................... .................................................... ...................... 62.2 Temperature .............................................. ..................................................... .................................................... .................................................... ...................... 62.3 Force and pressure .................................................... .................................................... .................................................... ..................................................... ....7

2.4 Heat, work, energy and power .............................................. ..................................................... .................................................... ....................................... 72.5 Substances and phase change .............................................. .................................................... ..................................................... ....................................... 82.6 Latent heat ................................................. ..................................................... .................................................... .................................................... ...................... 92.7 Superheat ................................................... ..................................................... .................................................... .................................................... ...................... 92.8 Refrigerant diagrams ............................................... .................................................... .................................................... ..................................................... . 10

3. Refrigerant circuit ........................................................................................................................................................................... 113.1 Evaporator .................................................. ..................................................... .................................................... .................................................... ................... 113.2 Compressor ............................................... ..................................................... .................................................... .................................................... ................... 113.3 Compressor, method of operation ................................................ .................................................... ..................................................... ........................... 113.4 Condenser .................................................. ..................................................... .................................................... .................................................... ................... 123.5 Expansion process ..................................................... .................................................... .................................................... ..................................................... . 123.6 High and low pressure sides of the refrigeration plant .....................................................................................................................................................12

4. Refrigeration process, pressure/enthalpy diagram ....................................................................................................... 13

5. Refrigerants ....................................................................................................................................................................................... 145.1 General requirements .............................................. .................................................... .................................................... ..................................................... . 145.2 Fluorinated refrigerants .................................................. ..................................................... .................................................... ............................................. 145.3 Ammonia NH3 .................................................... .................................................... ..................................................... .................................................... .......... 145.4 Secondary refrigerants .................................................... ..................................................... .................................................... ............................................. 14

6. Refrigeration plant main components ................................................................................................................................. 156.1 Compressor ............................................... ..................................................... .................................................... .................................................... ................... 156.2 Condenser .................................................. ..................................................... .................................................... .................................................... ................... 156.3 Expansion valve ................................................ .................................................... ..................................................... .................................................... .......... 176.4 Evaporation systems ................................................ .................................................... .................................................... ..................................................... . 18

7. The practical build-up of a refrigeration plant ................................................................................................................. 19


4/24


5/24

Lecture Refrigeration - an introduction to the basics


The job of a refrigeration plant is to cool articlesor substances down to, and maintain them at atemperature lower than the ambient tempera-ture. Refrigeration can be defined as a processthat removes heat.

The oldest and most well-known among refrige-rants are ice, water, and air. In the beginning, thesole purpose was to conserve food. The Chinesewere the first to find out that ice increased the lifeand improved the taste of drinks and for centu-ries Eskimos have conserved food by freezing it.

1. Introduction

Aa0

_0030_

00_

A1

Aa0_

0003_

00_

A1

heat heat

Aa0_

0002_

00_

A1

Aa0_

0005_

00_

A1

At the beginning of the last century, terms likebacteria, yeast, mould, enzymes etc. were known.It had been discovered that the growth of micro-organisms is temperature-dependent, thatgrowth declines as temperature falls, and that

growth becomes very slow at temperatures be-low +10 C.

As a consequence of this knowledge, it was nowpossible to use refrigeration to conserve food-stuffs and natural ice came into use for this pur-pose.

The first mechanical refrigerators for the produc-tion of ice appeared around the year 1860. In1880 the first ammonia compressors and insulat-ed cold stores were put into use in the USA.

Electricity began to play a part at the beginningof this century and mechanical refrigerationplants became common in some fields: e.g. brew-eries, slaughter-houses, fishery, ice production,for example.

After the Second World War the development ofsmall hermetic refrigeration compressors evolvedand refrigerators and freezers began to take theirplace in the home. Today, these appliances are re-garded as normal household necessities.

There are countless applications for refrigeration

plants now. Examples are:

Foodstuff conservationProcess refrigerationAir conditioning plantsDrying plantsFresh water installationsRefrigerated containersHeat pumpsIce productionFreeze-drying

Transport refrigeration

In fact, it is difficult to imagine life without airconditioning, refrigeration and freezing - their

impact on our existence is much greater thanmost people imagine.


6/24



2.1 Unit systems

2. Fundamental terms On an international level, agreement has beenreached on the use of the Systeme InternationaldUnits - often referred to as the SI-system. For anumber of countries the implementation of theSI-system is still an on-going process.

In this booklet the SI-system will be the primaryunit system used. However, in many parts of therefrigeration community it is still practice to usemetric units or other alternative units. Therefore,the practically used alternative units will be givenin parenthesis where needed.

The table shows the SI-units and the other oftenused alternative units for the quantities that areused in this booklet.

Quantity SI-unit Alternative units

Time s (second) h (hour)

Length m (meter) in (inch)ft (foot)

Mass kg (kilogram) lb (pound)

Temperature K (Kelvin) C (Celsius)F (Fahrenheit)

Force N (Newton) kp (kilopond)

Pressure Pa (Pascal) = N/m2 bar

atm (atmosphere)

mm Hg (millimeter mercu-ry column)

psi (pound per square inch)

Energy J (Joule) = Nm kWh (kilowatt hour)

cal (calorie)

Btu (British thermal unit)

Power W (Watt) = J/s calorie/h, Btu/h

Name pico nano micro mili kilo Mega Giga Tera Peta

Prefix p n m k M G T P

Factor 10-12 10-9 10-6 10-3 103 106 109 1012 1015

2.2 Temperature Temperature is a very central property in refriger-ation. Almost all refrigeration systems have thepurpose of reducing the temperature of anobject like the air in a room or the objects storedin that room.

The SI-unit for temperature Kelvin[K] is an abso-lute temperature because its reference point [0 K]is the lowest temperature that it in theory wouldbe able to obtain.

When working with refrigeration systems thetemperature unit degree Celsius [C] is a morepractical unit to use. Celsius is not an absolute

temperature scale because its reference point(0 C) is defined by the freezing point of water(equal to 273.15 K).

The only difference between Kelvin and Celsius isthe difference in reference point. This means thata temperature difference of 1 C is exactly thesame as a temperature difference of 1 K.

In the scientific part of the refrigeration commu-nity temperature differences are often described

using [K] instead of [C]. This practice eliminatesthe possible mix-up of temperatures and temper-ature differences.

The choice of prefix is free but the best choicewill normally be the one where the value writtenwill be in the range from 0.1 to 999.9.

Prefixes should not be used for combined SI-units- except when [kg] is used.

Example:2000 W/m2 K should be written as 2.000 103W/m2 K and not as 2 kW/m2 K.

The practical use of the SI-units is strongly associ-ated with the use of the decadic prefixes to avoidwriting either very small or large numbers. A partof the prefixes used can be seen in the table be-low.

Example:The atmospheric air pressure is 101325 Pa. Usingthe decadic prefixes from the table below thebest way of writing this would be 101.325 kPa.


7/24



Fundamental terms The SI-unit for force is Newton(N) which is actual-ly a [kg m/s2].

A man wearing skis can stand in deep snow with-out sinking very deep - but if he steps out of his

skis his feet will probably sink very deep into thesnow. In the first case the weight of the man isdistributed over a large surface (the skis). In thesecond case the same weight is distributed onthe area of his shoe soles - which is a much small-er area than the area of the skis. The differencebetween these two cases is the pressure that theman exerts on the snow surface.

Pressure is defined as the force exerted on anarea divided by the size of the area. In the exam-ple with the skier the force (gravity) is the same inboth cases but the areas are different. In the firstcase the area is large and so the pressure be-comes low. In the second case the area is small

and so the pressure becomes high.

2.3 Force and pressure

Aa0_

0006_

00_

A1

In refrigeration pressure is mostly associated withthe fluids used as refrigerants. When a substancein liquid or vapour form is kept within a closedcontainer the vapour will exert a force on the in-side of the container walls. The force of the va-pour on the inner surface divided by its area iscalled the absolute pressure.

For practical reasons the value for pressure issometimes stated as pressure above atmospher-ic pressure - meaning the atmospheric pressure

(101.325 kPa = 1.013 bar) is subtracted from theabsolute pressure. The pressure above atmos-pheric pressure is also often referred to as gauge

pressure.

The unit used should reflect the choice of abso-lute pressure or gauge pressure. An absolutepressure is indicated by the use of lowercase aand a gauge pressure is indicated by a lower-case g.

Example:The absolute pressure is 10 bar(a) which convert-ed to gauge pressure becomes (10 - 1.013) bar(g) 9 bar(g). The combination of the SI-unit forpressure [Pa] and the term gauge pressure is notrecommended.

Other units for pressure that are still used todayare mm of mercury column[mmHg], and meter wa-ter gauge[mwg]. The latter is often used in con-nection with pumps to indicate the height of the

water column that the pump is able to generate.

Vacuum is defined as an absolute pressure of 0 Pa- but since it is almost impossible to obtain thisthe term vacuum is used generally to describe apressure much lower than the atmospheric pres-sure. Example: The absolute pressure is 0.1 bar(a)which converted to gauge pressure becomes(0.1 - 1.013) bar(g) 0.9 bar(g). Vacuum is alsooften described in Torr(1 Torr is equal to 10 Pa)and millibar(a thousandth of a bar).

2.4 Heat, work, energy and

power

Heat and work are both forms of energy that can

be transferred between objects or systems. Thetransfer of heat is closely connected to the tem-perature (or temperature difference) that existsbetween two or more objects. By itself heat is al-ways transferred from an object with high tem-perature to objects with lower temperatures.Heating of water in a pot on a stove is a goodeveryday example of the transfer of heat. Thestove plate becomes hot and heat is transferredfrom the plate through the bottom of the pot andto the water. The transfer of heat to the watercauses the temperature of the water to rise. Inother words, heating an object is the same as trans-ferring energy (heat) to the object.

In many practical applications there is a need toreduce the temperature of an object instead ofincreasing it. Following the example above thiscan only be done if you have another object with

a lower temperature than that of the object you

wish to cool. Putting these two objects into con-tact will cause a transfer of heat away from theobject you wish to cool and, consequently, itstemperature will decrease. In other words, coolingan object is the same as transferring energy (heat)away from the object.

The transfer of work is typically connected to theuse of mechanical shafts like the one rotating inan electric motor or in a combustion engine.Other forms of work transfer are possible but theuse of a rotating shaft is the primary methodused in refrigeration systems.

As mentioned both heat and work are forms of en-

ergy. The methods of transfer between objects aredifferent but for a process with both heat and worktransfer it is the sum of the heat and work transferthat determines the outcome of the process.


8/24



Aa0_

0010_

00_

A1

Fundamental terms The SI-unit Joule[J] is used to quantify energy,heat and work. The amount of energy needed toincrease the temperature of 1 kg of water from 15to 16 C is 4.187 kJ. The 4.178 kJ can be trans-ferred as heat or as work - but heat would be the

most used practical solution in this case.

There are differences in how much energy is re-quired to increase the temperature of varioussubstances by 1 K. For 1 kg of pure iron app.0.447 kJ is needed whereas for 1 kg of atmos-pheric air only app. 1.0 kJ is needed. The propertythat makes the iron and air different with respectto the energy needed for causing a temperatureincrease is called the specific heat capacity. It isdefined as the energy required to cause a tem-perature increase of 1 K for 1 kg of the substance.

The unit for specific heat capacity is J/kg K.

The rate at which energy is transferred is called

power. The SI-unit for power is Watt (W).

Example:If 10 J is transferred per second, the rate of energytransfer is stated as 10 J/s = 10 W. In the SI-systemthe choice of unit for power is the same for transferof heat and work. In other unit systems the transfer

rates for heat and work could have different units.

All substances can exist in three different phases:solid, liquid, and vapour. Water is the most naturalexample of a substance that we use almost every-day in all three phases. For water the three phaseshave received different names - making it a bitconfusing when using it as a model substance.

The solid form we call ice, the liquid form we justcall water, and the vapour form we call steam.What is common to these three phases is that thewater molecules remain unchanged, meaningthat ice, water, and steam all have the same

chemical formula: H2O.

When taking a substance in the solid to the liq-uid phase the transition process is called meltingand when taking it further to the vapour phasethe transition process is called boiling (evapora-tion). When going in the opposite direction

2.5 Substances and phasechange

taking a substance from the vapour to the liquidphase the transition process is called condens-ing and when taking it further to the solid phasethe transition process is called freezing (solidifi-cation).

Aa0_

0009_

00_

A1

2.4 Heat, work, energy andpower (cont.)

At constant pressure the transition processes dis-play a very significant characteristic. When ice isheated at 1 bar its temperature starts rising untilit reaches 0 C - then the ice starts melting.During the melting process the temperature does

not change - all the energy transferred to themixture of ice and water goes into melting the iceand not into heating the water. Only when the icehas been melted completely will the furthertransfer of energy cause its temperature to rise.

The same type of behaviour can be observedwhen water is heated in an open pot. The water

temperature increases until it reaches 100 C -then evaporation starts. During the evaporationprocess the temperature remains at 100 C. Whenall the liquid water has evaporated the tempera-ture of the steam left in the pot will start rising.

The temperature and pressure a substance is ex-posed to determine whether it exists in solid, liq-uid, or vapour form - or in two or all three formsat the same time. In our local environment ironappears in its solid form, water in its liquid andgas forms, and air in its vapour form.


9/24



Aa0_

0013_

00_

A1

Fundamental terms

Going back to the process of ice melting it is im-portant to note that the amount of energy that

must be transferred to 1 kg of ice in order to meltit is much higher than the energy needed tochange the temperature of 1 kg of water by say1 K. In section 2.4 the specific heat capacity ofwater was given as 4.187 kJ/kg K. The energyneeded for melting 1 kg of ice is 335 kJ. The sameamount of energy that can melt 1 kg of ice canincrease the temperature of 1 kg of water by(335 kJ/4.187 kJ/kg K) = 80 K!

When looking at the boiling process of water theenergy needed for evaporating 1 kg of water is2501 kJ. The same amount of energy that canevaporate 1 kg of water can increase the tempe-rature of not 1 but 6 kg of water by 100 K!

These examples show that energy transfer relat-ed to the transitional processes between phasesis significant. That is also why ice has been usedfor cooling - it takes a lot of energy to melt the

2.6 Latent heat

ice and while the ice melts the temperature staysat 0 C.

The refrigerating effect in refrigeration systems isbased on the use and control of the phase transi-

tion processes of evaporation. As the refrigerantevaporates it absorbs energy (heat) from its sur-roundings and by placing an object in thermalcontact with the evaporating refrigerant it can becooled to low temperature.

Aa0_

0012_

00_

A1

Aa0_

0011_

00_

A1

Different substances have different melting andboiling points. Gold for example melts at 1064 C,chocolate at 26 C and most refrigerants melt attemperatures around -100 C!

For a substance that is present in two of its phas-es at the same time - or undergoing a phasechange - pressure and temperature become de-pendent. If the two phases exist in a closed con-tainer and the two phases are in thermal equilib-rium the condition is said to be saturated. If thetemperature of the two-phase mixture is in-creased the pressure in the container will also in-crease. The relationship between pressure andtemperature for saturated conditions (liquid and

vapour) is typically called the vapour pressurecurve. Using the vapour pressure curve one candetermine what the pressure will be for an evap-orating or condensing process.

2.5 Substances and phasechange (cont.)

Superheat is a very important term in the termi-nology of refrigeration - but it is unfortunatelyused in different ways. It can be used to describea process where refrigerant vapour is heatedfrom its saturated condition to a condition athigher temperature. The term superheat can alsobe used to describe - or quantify - the end condi-

tion of the before-mentioned process.

Superheat can be quantified as a temperature dif-ference - between the temperature measuredwith a thermometer and the saturation tempera-ture of the refrigerant measured with a pressuregauge. Therefore, superheat can not be deter-

2.7 Superheat mined from a single measurement of tempera-ture alone - a measurement of pressure or satura-tion temperature is also needed. When superheatis quantified it should be quantified as a tempera-ture difference and, consequently, be associatedwith the unit [K]. If quantified in [C] it can be thecause of mistakes where the measured tempera-

ture is taken for the superheat or vice versa.

The evaporation process in a refrigeration systemis one of the processes where the term superheatis used. This will be explained further in the nextchapter.


10/24



Fundamental terms The characteristics of a refrigerant can be illustrat-ed in a diagram using the primary properties asabscissa and ordinate. For refrigeration systemsthe primary properties are normally chosen asenergy content and pressure. Energy content is re-

presented by the thermodynamic property of spe-cific enthalpy - quantifying the change in energycontent per mass unit of the refrigerant as it un-dergoes processes in a refrigeration system. An ex-ample of a diagram based on specific enthalpy(abscissa) and pressure (ordinate) can be seen be-low. For a refrigerant the typically applicable inter-val for pressure is large - and therefore diagramsuse a logarithmic scale for pressure.

The diagram is arranged so that it displays the liq-uid, vapour and mixture regions for the refriger-ant. Liquid is found to the left (with a low energycontent) - vapour to the right (with a high energy

content). In between you find the mixture region.The regions are bounded by a curve - called thesaturation curve. The fundamental processes ofevaporation and condensation are illustrated.

The idea of using a refrigerant diagram is that itmakes it possible to represent the processes in therefrigeration system in such a way that analysisand evaluation of the process becomes easy.When using a diagram determining system oper-ating conditions (temperatures and pressures) sys-tem refrigerating capacity can be found in a rela-tively simple and quick manner.

Diagrams are still used as the main tool for analysisof refrigeration processes. However, a number ofPC programmes that can perform the same analy-sis faster and with more details have become gen-erally available.

2.8 Refrigerant diagrams

Aa0_

0046_

02_

A1


11/24



The physical terms for the refrigeration processhave been dealt with previously, even though forpractical reasons water is not used as a refrige-rant.

3. Refrigerant circuit

Aa0_

0021_

00_

A1

Aa0_

0022_

00_

A1

3.1 Evaporator

3.2 Compressor

A simple refrigerant circuit is built up as shown inthe sketch below. In what follows, the individualcomponents are described to clarify a final overallpicture.

A refrigerant in liquid form will absorb heat whenit evaporates and it is this conditional changethat produces cooling in a refrigerating process. Ifa refrigerant at the same temperature as ambientis allowed to expand through a hose with an out-let to atmospheric pressure, heat will be taken upfrom the surrounding air and evaporation will oc-cur at a temperature corresponding to atmos-pheric pressure.

If in a certain situation pressure on the outlet side(atmospheric pressure) is changed, a differenttemperature will be obtained since this is analo-

gous to the original temperature - it is pressure-dependent.

The component where this occurs is the evapora-

tor, whose job it is to remove heat from the sur-roundings, i.e. to produce refrigeration.

The refrigeration process is, as implied, a closedcircuit. The refrigerant is not allowed to expand tofree air.

When the refrigerant coming from the evapora-tor is fed to a tank the pressure in the tank willrise until it equals the pressure in the evaporator.

Therefore, refrigerant flow will cease and the tem-perature in both tank and evaporator will gradu-ally rise to ambient.

To maintain a lower pressure, and, with it a lowertemperature it is necessary to remove vapour.

This is done by the compressor, which sucks va-pour away from the evaporator. In simple terms,the compressor can be compared to a pump thatconveys vapour in the refrigeration circuit.

In a closed circuit a condition of equilibrium willalways prevail. To illustrate this, if the compressorsucks vapour away faster than it can be formed inthe evaporator the pressure will fall and with itthe temperature in the evaporator. Conversely, ifthe load on the evaporator rises and the refrige-rant evaporates quicker, the pressure and with it

the temperature in the evaporator will rise.

Refrigerant leaves the evaporator either as satu-rated or weak superheated vapour and enters thecompressor where it becomes compressed.Compression is carried out as in a petrol engine,i.e. by the movement of a piston.The compressorrequires energy and carries out work. This work istransferred to the refrigerant vapour and is calledthe compression input.

Because of the compression input, vapour leavesthe compressor at a different pressure and the extra

energy applied causes strong superheating of thevapour. Compression input is dependent on plantpressure and temperature. More work is of course

3.3 Compressor, method ofoperation

required to compress 1 kg vapour 10 bar than tocompress the same amount 5 bar.

Aa0_

0020_

02_

A1


12/24



The refrigerant gives off heat in the condenser,and this heat is transferred to a medium having alower temperature. The amount of heat given offis the heat absorbed by the refrigerant in theevaporator plus the heat created by compression

input.

The heat transfer medium can be air or water, theonly requirement being that the temperature islower than that which corresponds to the condens-ing pressure. The process in the condenser can oth-erwise be compared with the process in the evapo-rator except that it has the opposite sign, i.e. theconditional change is from vapour to liquid.

3.4 Condenser

3.5 Expansion process Liquid from the condenser runs to a collectingtank, the receiver. This can be likened to the tankmentioned under section 3.1 on the evaporator.

Pressure in the receiver is much higher than thepressure in the evaporator because of the com-pression (pressure increase) that has occurred inthe compressor. To reduce pressure to the samelevel as the evaporating pressure a device mustbe inserted to carry out this process, which iscalled throttling, or expansion. Such a device istherefore known either as a throttling device oran expansion device. As a rule a valve is used - athrottle or expansion valve.

Ahead of the expansion valve the liquid will be alittle under boiling point. By suddenly reducingpressure a conditional change will occur; the liq-

Aa0_

0045_

02_

A1

Aa0_

0035_

02_A

1

uid begins to boil and evaporate. This evapora-tion takes place in the evaporator and the circuitis thus complete.

There are many different temperatures in-volved in the operation of a refrigerationplant since there are such things as sub-cooled liquid, saturated liquid, saturated va-pour and superheated vapour. There are how-ever, in principle, only two pressures; evapo-rating pressure and condensing pressure. Theplant then is divided into high pressure andlow pressure sides, as shown in the sketch.

3.6 High and low pressure sidesof the refrigeration plant

Aa0_

0034_

02_

A1


13/24



The condensed refrigerant in the condenser is incondition A which lies on the line for the boilingpoint of the liquid. The liquid has thus a tempera-ture tc, a pressure pc also called saturated temper-ature and pressure.

The condensed liquid in the condenser is furthercooled down in the condenser to a lower temper-ature A1 and now has a temperature tl and an en-thalpy h0. The liquid is now sub-cooled whichmeans that it is cooled to a lower temperaturethan the saturated temperature.

The condensed liquid in the receiver is in condi-tion A1 which is sub-cooled liquid. This liquidtemperature can change if the receiver and liquidis either heated or cooled by the ambient tem-perature. If the liquid is cooled the sub-coolingwill increase and visa versa.

When the liquid passes through the expansionvalve its condition will change from A1 to B. Thisconditional change is brought about by the boil-ing liquid because of the drop in pressure to p0.At the same time a lower boiling point is pro-duced, t0, because of the drop in pressure.

In the expansion valve the enthalpy is constanth0, as heat is neither applied nor removed.

At the evaporator inlet, point B, there is a mixtureof liquid and vapour while in the evaporator at Cthere is saturated vapour. At the evaporator outlet

4. Refrigeration process,pressure/enthalpydiagram

point C1 there is super-heated vapour whichmeans that the suction gas is heated to a highertemperature than the saturated temperature.Pressure and temperature are the same at point Band at outlet point C1 where the gas is super-heat-

ed the evaporator has absorbed heat from thesurroundings and the enthalpy has changed to h1.

When the refrigerant passes through the com-pressor its condition changes from C1 to D.Pressure rises to condensing pressure pc. Thetemperature rises to thot-gas which is higher thanthe condensing temperature tc because the va-pour has been strongly superheated. More ener-gy (from the electrical motor) in the form of heathas also been introduced and the enthalpy there-fore changes to h2.

At the condenser inlet, point D, the condition isthus one of superheated vapour at pressure pc.

Heat is given off from the condenser to the sur-roundings so that the enthalpy again changes tomain point A1. First in the condenser there occursa conditional change from strongly superheatedvapour to saturated vapour (point E), then a con-densation of the saturated vapour. From point Eto point A the temperature (condensing tempera-ture) remains the same, in that condensation andevaporation occurs at constant temperature.From point A to point A1 in the condenser thecondensed liquid is further cooled down, but thepressure remains the same and the liquid is nowsub-cooled.

tc = condensing temperaturepc = condensing pressure

tl = liquid temperature

t0 = evaporating temperature

p0 = evaporating pressure


14/24



5. Refrigerants During the examination of the refrigeration processthe question of refrigerants was not discussed sinceit was not necessary to do so in connection with thebasic physical principles of the conditional changeof substances. It is well known, however, that in

practice different refrigerants are used according tothe specific application and requirements. The mostimportant factors are as follows:

The refrigerant ought not to be poisonous.Where this is impossible, the refrigerant musthave a characteristic smell or must contain atracer so that leakage can quickly be observed.

The refrigerant ought not to be flammablenor explosive. Where this condition cannotbe met the same precautions as in the firstpoint must be observed and local legisla-tions must be followed.

The refrigerant ought to have reasonablepressure, preferably a little higher than at-

mospheric pressure at the temperatures re-quired to be held in the evaporator.

5.1 General requirements

5.2 Fluorinated refrigerants Fluorinated refrigerants always carry the designa-tion R followed by a number, e.g. R22, R134a,R404A and R407C. Sometimes they are met bear-ing their trade names. The fluorinated refrigerantsall have the following features:

Vapour is smell-free and non-irritant.Extensively non-poisonous. In the presenceof fire the vapour can give off fluoric acidand phosgene, which are very poisonous.

Non-corrosive.Non-flammable and non-explosive.

The most common fluorinated refrigerants are:

R134a, which is a substance of the ethane groupwith the formula CH2FCF3 and has a normal boilingpoint of 26.1 C. Its thermodynamic propertiesmake it suitable as a refrigerant for medium temper-ature applications such as domestic refrigerators.

R22, which is a substance of the methane groupwith the formula CHF2CI and has a boiling point of40.8 C. Its thermodynamic properties make itsuitable as a refrigerant for a wide range of applica-

tions in commercial refrigeration and air condition-ing. R22 is being phased out as refrigerant in manycountries due to its ozone depleting potential.

R404A/R507A (also known as R507), which is amixture of the refrigerants R125 (CHF2CF3) and

R143a (CH3CF3) with a boiling point at (46.7 C)which is slightly lower than for R22. Its thermody-namic properties makes it suitable as a refrigerantfor low and medium temperature applications incommercial refrigeration (e.g. supermarkets).

R407C, which is a mixture of the refrigerants R32(CH2F2), R125 (CHF2CF3) and R134a (CH2FCF3) witha boiling point at (43.6 C) which is slightly lowerthan for R22. Its thermodynamic properties make

it suitable as a refrigerant for medium and hightemperature applications in residential and com-mercial air conditioning.

R410A, which is a mixture of the refrigerants R32(CH2F2) and R125 (CHF2CF3) with a boiling point at(51.4 C) which is lower than for R22. I ts thermo-dynamic properties make it suitable as a refriger-ant for medium and high temperature applicationsin residential and commercial air conditioning.

Apart from these fluorinated refrigerants there isa long series of others not seen very often today:R23, R123, R124 and R218.

Except for R22, systems with fluorinated hydro-carbons are in general lubricated with polyol es-ter oils (POE). These oil types are much more sen-sitive to react chemically with water, the so-calledhydrolysis reaction. For that reason systems to-day are kept extremely dry with filter driers.

Ammonia NH3 is used extensively in large indus-trial refrigeration plants. Its normal boiling pointis 33 C. Ammonia has a characteristic smelleven in very small concentrations in air. It cannot

5.3 Ammonia NH3 burn, but it is moderately explosive when mixedwith air in a volume percentage of 13 to 28%.Because of corrosion, copper or copper alloysmust not be used in ammonia plants.

The refrigerants mentioned above are often de-signated primary refrigerants. As an interme-diate link in heat transmission from the surround-

5.4 Secondary refrigerants ings to the evaporator, the so-called secondaryrefrigerants can be used, e.g. water, brine, atmos-pheric air etc.

To avoid heavy refrigerator design the pres-sure, which corresponds to normal con-densing pressure, must not be too high.Relatively high evaporating temperature isrequired so that heat transmission can occur

with least possible circulating refrigerant.Refrigerant vapour ought not to have toohigh a specific volume because this is a de-terminant for compressor stroke at a partic-ular cold yield.

The refrigerant must be chemically stable atthe temperatures and pressures normal in arefrigeration plant.

The refrigerant ought not to be corrosiveand must not, either in liquid or vapourform, attack normal design materials.

The refrigerant must not break down lubri-cating oil.

The refrigerant must be easy to obtain andhandle.

The refrigerant must not cost too much.


15/24



The job of the compressor is to suck vapour fromthe evaporator and force it into the condenser.

The most common type is the piston compressor,but other types have won acceptance, e.g. cen-trifugal scroll and screw compressors.

The piston compressor covers a very large capac-ity range, right from small single cylinder modeIsfor household refrigerators up to 8 to 12 cylindermodeIs with a large swept volume for industrialapplications.

In the smallest applications the hermeticcompressor is used, where compressor andmotor are built together as a complete hermeticunit.

For medium sized plants one of the mostcommon compressors is the larger sizes ofhermetic compressors in either piston or scroll

versions. The applications are both airconditioning, general commercial refrigerationand chillers.

For larger plants the most common is the semi-hermetic compressor. The advantage here is thatshaft glands can be avoided; these are verydifficult to replace when they begin to leak.However, the design cannot be used on ammoniaplants since this refrigerant attacks motorwindings.

Still larger HFC compressors, and all ammoniacompressors, are designed as open compres-sors, i.e. with the motor outside the crankcase.

Power transmission can be direct to the crank-shaft or through a V-belt drive.

For quite special applications there is the oil-freecompressor. But lubrication of bearings and cylin-der walls with oil is normally always necessary.On large refrigeration compressors oil is circulat-ed by an oil pump.

6. Refrigeration plantmain components

Aa0_

0004_

00_

A1

Aa0_

0038_

00_A

1

Aa0_

0029_

00_

A1

6.1 Compressor

Aa0_

0031_

00_A

1

Aa0_

0032_

00_

A1

The purpose of the condenser is to remove theamount of heat that is equal to the sum of theheat absorbed in the evaporator and the heatproduced by compression. There are many diffe-

rent kinds of condenser.

Shell and tube condenser. This type of condenser isused in applications where sufficient cooling wateris available. It consists of a horizontal cylinder withwelded-on flat end caps, which support the cool-ing tubes. End covers are bolted to the end plates.

The refrigerant condensate flows through thecylinder, the cooling water through the tubes.

The end covers are divided into sections by ribs.The sections act as reversing chambers for the

6.2 Condenser

water so that it circulates several times throughthe condenser. As a rule, the water becomesheated 5-10 C when it has passed through acondenser.


16/24



If it is desirable or necessary to cut down on theamount of water an evaporating condensercan beused instead. This type of condenser consists of ahousing in which there is a condensing coil, waterdistribution tubes, deflection plates and fans.

The warm refrigerant vapour is led to the top ofthe condensing coil after which it condensesand runs from the bottom of the coil as liquid.

Refrigeration plant maincomponents

6.2 Condenser (cont.)

Water distribution tubes with nozzles are placedover the condensing coil so that water is spreadover and runs down the coil. The fans direct astrong flow of air across the condensing coil.When the falling drops of water meet the up-

ward air flow some of the water will evaporate.This absorbs the necessary evaporating heatfrom the refrigerant vapour and causes it to con-dense.

1. Fan

2. Deflector plate

3. Outer covering

4. Superheat remover

5. Condenser tubing

6. Air intake

7. Collecting tray

8. Overflow pipe

9. Water distribution pipe10. Water circulation pump

11. Air intake

1. Fan

2. Deflector plate

3. Outer covering

4. Nozzle

6. Air intake

7. Collecting tray

8. Overflow pipe

9. Cooling water from condenser

10. Air intake

11. Cooling water return to condenser

The principal involving water evaporation isalso used in connection with cooling towers.

These are installed when the most practical isto place a shell and tube condenser near acompressor. The water is then circulated in a

circuit between condenser and cooling tower.

In principle, the cooling tower is built upthe same as the evaporating condenser,

but instead of condensing elements thereare deflector plates. Air is heated on its waythrough the tower by direct contact with thetrickle of water travelling downwards andis, therefore, able to absorb an increasing

amount of moisture coming from part-evaporation. In this way, the cooling waterloses heat. Water loss is made up by supplyingmore water.


17/24



It is possible to save 90-95% water consumptionby using evaporating condensers or coolingtowers, when compared to the water consump-tion of shell and tube condensers.

For one reason or another it is not always pos-sible to use water for the condensing process. Insuch cases an air-cooled condensermust beused. Since air has poor heat transfer characte-ristics, compared with water, a large surface onthe outside of the condensing tubes is neces-sary. This is achieved using large ribs or fins and,in addition, by ensuring generous air circulationmechanically.

This is the normal condenser for commercial re-frigeration.

Aa0_

0039_

00_

A1

The main purpose of the expansion valve is to en-sure a sufficient pressure differential between thehigh and low pressure sides of the plant. The sim-plest way of doing this is to use a capillary tubeinserted between the condenser and evaporator.

The capillary tube is, however, only used insmall, simple appliances like refrigerators be-

Aa0_

0041_

00_

A1

Aa0_

0040_

00_

A1


6.2 Condenser (cont.)

6.3 Expansion valve cause it is not capable of regulating the amountof liquid that is injected into the evaporator. Aregulating valve must be used for this process,the most usual being the thermostatic expan-sion valve, which consists of a valve housing,capillary tube and a bulb. The valve housing isfitted in the liquid line and the bulb is fitted onthe evaporator outlet.

This figure shows an evaporator fed by a thermo-static expansion valve. A small amount of liquid is

contained in a part of the bulb. The rest of thebulb, the capillary tube and the space above thediaphragm in the valve housing is charged withsaturated vapour at a pressure corresponding tothe temperature at the bulb. The space under thediaphragm is in connection with the evaporatorand the pressure is therefore equal to the evapo-rating pressure.

1. Inlet with strainer

2. Cone

3. Outlet

4. Bore

5. Connection for pressure equalizing6. Spring housing

7. Diaphragm

8. Capillary tube

9. Spindle for setting spring pretension

(opening superheat)

10. Bulb


18/24



The degree of opening of the valve is determinedby:

The pressure produced by the bulb temper-ature acting on the top surface of the dia-phragm.

The pressure under the diaphragm, which isequal to the evaporating pressure.The pressure of the spring acting on the un-derside of the diaphragm.

During normal operation, evaporation will ceasesome distance up in the evaporator.

Then, saturated gas appears which becomes su-perheated on its way through the last part of theevaporator. The bulb temperature will thus beevaporating temperature plus superheat, e.g. at

Finned evaporator

Plain tube evaporator

Ribbed tube evaporator

Aa0_

0042_

00_

A1

Aa0_

0043_

00_

A1

Aa0_

0044_

00_

A1

6.4 Evaporation systems


6.3 Expansion valve (cont.)

10 C evaporating temperature the bulb tem-perature could be 0 C.

If the evaporator receives too little refrigerant thevapour will be further superheated and the tem-

perature at the outlet pipe will rise. The bulb tem-perature will then also rise and with it the vapourpressure in the bulb element since more of thecharge will evaporate. Because of the rise in pres-sure the diaphragm becomes forced down, thevalve opens and more liquid is supplied to theevaporator. Correspondingly, the valve will closemore if the bulb temperature becomes lower.

Thermostatic expansion valves are produced inseveral versions and of course there are manyvariants within each type.

Depending on the application, various require-ments are imposed on the evaporator. Evapora-tors are therefore made in a series of differentversions.

Evaporators for natural air circulation are used lessand less because of the relatively poor heat trans-fer from the air to the cooling tubes. Earlier ver-sions were fitted with plain tubes, but now it iscommon to use ribbed tubes or finned elements.

Evaporator performance is increased significantlyifforced air circulation is used. With an increase ofair velocity the heat transfer from air to tube isimproved so that for a given cold yield a smaller

evaporator surface than for natural circulationcan be used.

As the name implies, a chillercools down liquid.The simplest method is to immerse a coil of tubein an open tank. Closed systems are coming intouse more and more. Here, tube coolers made simi-lar to shell and tube condensers are employed.


19/24



Figure A shows the principle build-up of a refrig-eration plant for a simple cold store - much likethose that can be seen in butchers shops and su-permarkets.

The compressor unit can, for example, be in-stalled in an adjacent storage room with an outletto fresh air. Such a unit consists of a compressordriven by V-belt and electric motor. Additionally,the base frame carries an air-cooled condenserand a receiver. A fan is mounted on the shaft ofthe electric motor to force air through the con-denser and ensure the necessary degree of cool-ing. The line between compressor and condenseris known as the discharge line.

Today the majority of compressors used are ofthe semi-hermetic and hermetic types.

7. The practical build-upof a refrigeration plant

From the receiver, an uninsulated line, the liquidline, is taken out to the cold store where it is con-nected to the thermostatic expansion valve atthe evaporator inlet. The evaporator is built upwith close-pitch fins attached to tubes. It is also

equipped with a fan for forced air circulation anda drip tray.

From the outlet side of the evaporator a line,the suction line, is led back to the compressor.

The diameter of the suction line is somewhatlarger than the liquid line because it carriesvapour. For this reason the suction line is as arule insulated.

Thermostatic expansion valve

Fan

Evaporator

Drip tray

Cold store

0-2 C

Liquid line Pressure line

Suction lineCondenser

Compressor

Base frame

Compressor unit

Aa0_

0047_

00

Fig. A


20/24



7.1 bar 1.0 bar

7.6 bar7.6 bar

7.6 bar 0.8 bar

10 C

2 C

+2 C

0 C

+1 C

+2 C

+ 4 C

10 C

+27 C

+32 C

+34 C

+34 C

+23 C

+60 C

+2 C

+24 C

1.0 bar

tsuct.

= 12.5 C

Aa0_

0046_

00

Figure B gives details of momentary temperaturesin a refrigeration plant. At the compressor outletthe pressure is 7.6 bar and the temperature is 60 Cbecause of the presence of superheated gas. Thetemperature in the upper part of the condenser

will quickly fall to saturation temperature, whichat the pressure concerned will be 34 C, becausesuperheat is removed and condensation begins.

Pressure at the receiver outlet will remain more orless the same, while subcooling of the liquid be-gins because the temperature has fallen by 2 Cto 32 C.

In the evaporator a pressure of 1 bar and an evap-orating temperature of 10 C are indicated. In thelast part of the evaporator the vapour becomessuperheated so the temperature at the thermo-

static expansion valve bulb becomes +2 C, corre-sponding to the superheat set on the valve.

As illustrated below, air temperature will vary, inthat the air will take up heat on its way round the

store from products, walls, ceiling, etc. The tem-perature of the air blown across the condenserwill also vary with the time of year.

A refrigeration plant must then be dimensionedaccording to the largest load it will be subjectedto. To be able to accommodate smaller loads, fa-cilities must exist in the plant for altering yield.

The process of making such alterations is calledregulation and it is precisely regulation thatDanfoss automatic controls are made for. But thatis a subject, which is outside the scope of thispublication.

The practical build-up of arefrigeration plant

R134aFig. B


21/24

21

Notes


22/24

22

Notes


23/24

23

Notes


24/24

Appliance ControlsGeneral temperature controls for the home applianceindustry. The product range comprises CFC-free elec-

tromechanical and electronic thermostats for refrigera-tors and freezers produced to customer specificationsas well as service thermostats for all refrigeration andfreezing appliances.

Commercial CompressorsLarge hermetic reciprocating and scroll compressortechnologies for commercial air conditioning andrefrigeration. The compressors and condensing unitsare used in a large array of applications in bothbusinesses. This ranges from water chillers, largepackaged air conditioners as well as medium and low

temperature refrigeration systems for food storage andprocessing.

Danfoss CompressorsHermetic compressors and fan-cooled condensing unitsfor refrigerators, freezers and light commercial applica-tions such as bottle coolers and display counters. Danfossalso produces compressors for heating pump systemsas well as 12 and 24 volt compressors for refrigeratorsand freezers used in mobile applications and solarpower. The division has a leading position within en-

ergy utilisation, noise filtering and know-how aboutenvironment-friendly compressors.

The Danfoss product range for therefrigeration and air conditioning industry

Refrigeration and air conditioning controlsA comprehensive and highly reputed range of self-act-ing valves, electronic valves and regulators as well as

system protectors and line components for the refrig-eration and air conditioning market. These productsinclude thermostatic expansion valves, solenoid valves,thermostat and pressure controls, modulation pressureregulators, filter driers, shut-off valves, sight glasses,check valves, non-return valves and water valves.Decentralised electronic systems for full regulation andcontrol of refrigeration applications are also developedand produced at Danfoss.

Industrial ControlsProducts and customer specific solutions for industrial

monitoring and controls systems based on the principlesof pressure and temperature measurement, electricalpower and fluid control. Products include a wide rangeof automatic controls for process control and regulationsuch as contactors and motor starters, electrically, pneu-matically and temperature activated valves as well astemperature and pressure transmitters and switches.

danfoss refrigeration basics

Documents